is it possible to open up 2 windows at the same time to have for example inbox in first window and calendar in second one?
Unfortunately, no.
As Jay says, it is not possible. However, the Agenda Sidebar goes a long way to filling that need providing you with an overview of your calendars to the right of your messages.
If it is not visible, choose Menu > View > Sidebar > Regularly Sized Sidebar. The click on the Agenda Tab.
I had exact same question - perhaps this could be considered for a feature update. The ability to see an email at the same time as updating a calendar entry would be very helpful for copying details etc or even drag and drop email into the appointment notes.
Also at present when you switch between inbox and calendar tabs (to do the above) each time it goes back to todays date and most recent email meaning you have to find both again.
Yes separate windows in EMClient would be great. I run my EMClient Gmail Calendar in a separate Browser window. Its works for now.
I agree a separate window in em Client should be standard. In this way it is not usable for me and I have to go back to Outlook 